1. Field of the Invention
The present invention relates to an optical coherence tomography system and optical coherence tomography method for observing an image of internal structure of an object below its surface or a tomogram of lower epidermis of a biomedical tissue.
2. Discussion of the Related Art
In recent years, with the advancement of medical technique such as endoscopic therapy, there is a demand for a method of diagnosing pathological tissues non-invasively in real time. Conventionally, for example, an electronic endoscope using a CCD and imaging using CT, MRI and ultrasonic waves are used as diagnosis methods. However, the use of electronic endoscope is limited to observation of the surface of a living body. The image diagnosis system using CT, MRI or ultrasonic waves has a technical limitation in observing a target with resolution of the order of micron. To solve such problems, the light interference tomographic measurement technique has attracted attention.
The interference tomographic measurement technique has two types: time domain tomographic measurement and frequency domain tomographic measurement. The frequency domain tomographic measurement has two types: a spectrometer type and tunable light source type. Handbook of Optical Coherence Tomography, p 41-43, Mercel Dekker, Inc. 2002 discloses a measurement system using a tunable light source. The system irradiates a living body with light, continuously changes the wavelength of irradiation light, allows reference light to interfere the light with reflected light returned from a different depth in the living body with an interferometer and analyzes a frequency component of the interference signal to obtain a tomogram. This technique is expected as an advanced system because a tomogram of extreme high resolution can be created based on frequency analysis of the signal sent from the inside of the object. This system is suitable for practical use such as endoscope in that it has high sensitivity of measurement and is resistant to dynamic noise. As wavelength scanning band of the reflection light is larger, frequency analysis band is increased and thus, the resolution in a depth direction is increased.
In an optical coherence tomography device, it is necessary to obtain many points at a regular frequency interval according to resolution of the image in one wavelength scanning and set them as timing signals for Fourier transforming. The timing signal is generally referred to as a k trigger. An interval of a trigger signal corresponds to the range of observed depth, and as the interval is smaller, deeper analysis can be performed. The interval of a trigger signal must be a regular frequency interval. If the interval of the trigger signal is not an equal frequency interval, this causes a problem in that wavelength scanning becomes nonlinear resulting in generating a image with distortion or noise.
The normal wavelength-tunable laser light source, however, has a defect that wavelength does not linearly vary with respect to time, and thus, the trigger signal cannot be easily obtained at a regular frequency interval. Conventionally, as disclosed in R. Huber. et al. “Three-dimensional and C-mode OCT imaging with a compact, frequency swept laser source at 1300 nm”, 26 Dec. 2005/Vol. 13, No. 26/OPTICS EXPRESS 10523, in addition to an interferometer for causing interference with back scattered light from a sample, an optical system for generating the interference signal, such as other interferometers and Fabry-Perot etalon is annexed. It is common to carry out an interference processing of the sample and frequency calibration of the interference signal of the actual sample through the annexed optical system.
The device disclosed in the latter document, however, has problems that the annexed optical system is expensive and an expensive A/D board is required.